Semiconductor copper bond pad surface protection

ABSTRACT

Electronic packages with uninsulated portions of copper circuits protected with coating layers having thicknesses that are suitable for soldering without fluxing and are sufficiently frangible when being joined to another metal surface to obtain metal-to-metal contact between the surfaces.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.10/153,451 filed May 22, 2002, now patented, which, in turn, is aDivisional of U.S. patent application Ser. No. 09/412,542 filed Oct. 5,1999, now U.S. Pat. No. 6,413,576, which, in turn, claims priority fromU.S. Provisional Patent Application Nos. 60/103,032 filed Oct. 5, 1998,60/127,249 filed Mar. 31, 1999, and 60/146,674 filed Aug. 2, 1999. The542 Application is also a Continuation-In-Part of U.S. patentapplication Ser. No. 09/330,906 filed Jun. 11, 1999, now U.S. Pat. No.6,352,743, which also claims priority benefit under 35 U.S.C. § 119(e)of said U.S. Provisional Patent Application Nos. 60/103,032 filed Oct.5, 1998 and 60/127,249 filed Mar. 31, 1999. The disclosures of all sixapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to methods for protecting semiconductorcopper bond pad surfaces with ceramic coatings that are sufficientlyfrangible during ball, wedge or flip chip bonding to obtainmetal-to-metal contact between the bonding surfaces and the wires bondedthereto. The method protects the copper bond pads during extendedexposure to water and water solutions such as are experienced duringsawing.

The use of copper bond pads on semiconductor devices would be anattractive alternative to that of aluminum, were it not for atmosphericcontamination of the copper surface, which oxidizes readily to form acoating that is not removable by standard methods of wire bondingmachines, and requires the use of fluxes in solder-type interconnects,e.g., flip chip bonding. Present attempts to overcome this probleminvolve the use of a cover gas that is unavoidably expensive and complexand restricts bond head and work holder movement, or the use of a noblemetal or overplating with inert metals which are more costly and canlead to the formation of unwanted intermetallic compounds at the bondpad interface.

U.S. Pat. No. 5,771,157 encapsulates a wedge bond of an aluminum wire toa copper pad with the resin, after the bond is formed. No protectionagainst oxidation is provided to the copper pad prior to wedge bonding.

U.S. Pat. No. 5,785,236 protects a copper bond pad from oxidation with asurface layer of aluminum. This detracts from the advantages sought tobe obtained by replacing aluminum bond pads with copper bond pads.

There remains a need for methods by which copper bond pad surfaces maybe protected from oxidation prior to wire bonding or flip chipsoldering.

SUMMARY OF THE INVENTION

This need is met by the present invention. Ceramic coatings have nowbeen developed for the bonding surfaces of copper bond pads that aresufficiently frangible to obtain metal-to-metal contact between thebonding surface and the wire bonded thereto during ball or wedge wirebonding, and to obtain a surface suitable for soldering without fluxing.

It has also been discovered that the same ceramic coatings can begenerally used to protect the copper surfaces of electronic packages.That is, the present invention provides ceramic coatings for theprotection of the copper surfaces of organic substrate packages, metalsubstrate packages ceramic substrate packages, and the like.

According to one aspect of the present invention, a method is providedfor protecting the surface of an uninsulated portion of a copper circuitfrom environmental contamination detrimental to joining the surface toanother metal surface, wherein the method includes the step of coatingthe surface with a layer of a ceramic material having a thickness thatis suitable for soldering without fluxing and that is sufficientlyfrangible when the surfaces are being joined to obtain metal-to-metalcontact between the surfaces.

The invention is particularly suited to protecting the bonding surfacesof copper bond pads. Therefore, in a preferred embodiment of the presentinvention, the uninsulated portion of the copper circuit is the bondingsurface of a copper semiconductor bond pad.

The present invention thus provides electronic packages havinguninsulated copper circuit surfaces with coating layers that are capableof being removed at bonding or soldering. Therefore, according toanother aspect of the present invention, an electronic package isprovided containing at least one uninsulated copper surface coated witha layer of a ceramic material having a thickness that is suitable forsoldering without fluxing and which provides the layer with theaforementioned hardness. In a preferred embodiment, the electronicpackage is a semiconductor with uninsulated copper bond pads.

This aspect of the present invention includes electronic packages havinguninsulated portions of copper circuits coated with a layer of rareearth metals that form complexes with copper. The layer has a thicknessthat, upon formation of the copper complex and exposure to a reducingenvironment containing hydrogen, forms a ceramic hydride layer having athickness that is suitable for soldering without fluxing and whichprovides the layer with the aforementioned hardness.

This aspect of the present invention thus also includes electronicpackages having uninsulated portions of copper circuits with protectiveceramic metal hydride coatings. Therefore, according to another aspectof the present invention, an electronic package is provided containingan uninsulated portion of a copper circuit coated with a surface layerof a metal hydride compound selected from metal hydrides of copper-rareearth metal complexes and metal hydrides of copper-immiscible metalsthat form metal hydrides, in which the surface layer has a thicknessthat is suitable for soldering without fluxing and which provides thelayer with the aforementioned hardness. Again the preferred electronicpackage is a semiconductor having at least one copper bond pad.

The inventive method provides the ability to bond wires to coppercircuits using existing equipment without modification of the wirebonder, and without additional costs and limitations of cover gastechnology and hardware. The foregoing and other objects, features, andadvantages of the present invention are more readily apparent from thedetailed description of the preferred embodiments set forth below, takenin conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The sole drawing FIGURE is a schematic diagram of one method accordingto the present invention, in which semiconductor devices according tothe present invention having copper bond pads with bonding surfacescoated with hydride-forming materials and metal hydrides are alsodepicted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention forms protective ceramic coating layers on coppercircuit bonding surfaces of electronic packages, with thicknesses thatare suitable for soldering without fluxing. The ceramic layer thicknessis selected to provide at least the minimum hardness required for thelayer to be sufficiently frangible during ball or wedge wire bonding toobtain metal-to-metal contact between each bonding surface and the wirebonded thereto.

Ceramic, rather than metallic, coatings are employed because metalliclayers would be ductile and would plastically deform under impact.Because ceramic materials cannot be deformed in the plastic region,impact shatters the layer and allows it to be pushed aside during wirebonding.

Essentially all commonly used ceramic materials have a hardness suitablefor use with the present invention. One measure of ceramic hardness isthe Rockwell Superficial Hardness Scale (45-N) which is defined inSomiya, Advanced Technical Ceramics (Prentice Hall, Englewood Cliffs,N.J. 1996). Ceramic materials suitable for use with the presentinvention have a Rockwell Hardness (N-45) greater than about 38.

For purposes of the present invention, the meaning of the term ceramicmaterials is adopted as it is defined in Callister, Materials Scienceand Engineering, An Introduction (3rd Ed., John Wiley & Sons, New York1994), page 4. Callister defines ceramic materials as compounds betweenmetallic and nonmetallic elements that are most frequently oxides,nitrides and carbides. Ceramic materials within this classificationinclude materials composed of clay minerals, cement and glass. Ceramicmaterials are insulative to the passage of electricity and heat, and aremore resistant to high temperatures and harsh environments than metalsand polymers. As for mechanical behavior, ceramic materials are hard butvery brittle.

One method and apparatus of the present invention is depicted in thesole drawing FIGURE, in which bonding surface 12 of copper bond pad 10of a semiconductor device (not shown) is cleaned (Stage I). If thecopper surface is fresh and has not been exposed to a contaminatingatmosphere, Stage I cleaning is not required. In the depictedembodiment, the bonding surface 12 is coated with a layer 14 of ahydride-forming copper-immiscible metal or copper-complexing rare earthmetal (Stage II). For proper coating of surface layer 14, it isnecessary to reduce the oxides, hydroxides and sulfides that form on thesurface 12 of pad 10. Only after this reduction is complete can propersurface coating be performed. The surface 12 can be reduced by exposureto a reducing atmosphere, such as an atmosphere containing hydrogen, orby essentially any other conventional surface reducing techniques,including cleaning techniques such as plasma cleaning.

Examples of metals that are completely immiscible in copper include, butare not limited to, Ta, V and Nb. Examples of rare earth metals thatcomplex with copper include, but are not limited to, La, Y, and Ce.

The surface 12 of copper pad 10 is coated with metal layer 14 byconventional vapor deposition or analogous techniques. The rare earthmetals may require a heating step after deposition for the coppercomplex to form.

Surface layers of copper-immiscible hydride-forming metals can be formedby an alternative route. The copper-immiscible metal may be co-depositedwith the copper as the copper bond pads are formed during waferfabrication. By heating the wafers after fabrication, the co-depositedimmiscible metal will migrate to the surface of the copper bond pad,forming an oxidation-protective layer. Electroless or electrodepositiontechniques may also be employed.

The deposited layer 14 should be of a thickness capable of forming afrangible hydride layer. That is, the resulting ceramic layer shouldhave a thickness sufficient to provide the layer with a RockwellHardness (N-45) greater than about 38. Suitable ceramic layers have athickness between about 10 and about 1,000 angstroms, with a thicknessbetween about 25 and about 500 angstroms being preferred.

When a rare earth metal is employed, it is preferably deposited in alayer thin enough to form an essentially pure copper complex. This canbe accomplished using rare earth metal layers with thickness from about10 to about 1,000 Angstroms.

Copper-immiscible metal layers are preferably thin enough to be costcompetitive and permit ease of fabrication. For these purposes, thelayer 12 should be no thicker than 1/10 the total combined thickness ofthe pad 10 and layer 14. A thickness from about 10 to about 1,000Angstroms is preferred.

Layer 14 is then converted to a hydride layer (Stage III) by reductionwith hydrogen, either by heating the bond pad in an atmospherecontaining hydrogen, or by exposing the bond pad to ahydrogen-containing plasma, e.g, plasma-cleaning operations. Onceformed, the hydride layer 16 is stable at room temperature. It is notnecessary for deposition or hydride conversion of the layer 14 to beperformed at the time of wafer fabrication. Both processes can be doneat a later time. As noted above, for proper deposition of the layer 14,the surface 12 of bond pad 10 must be cleaned prior to deposition.

The hydride-formation step can take place at any stage prior to wirebonding or flip chip bonding, so long as the reducing environment issufficiently aggressive enough to reduce the layer 14 to remove anyatmospheric contamination. Suitable reducing conditions can be readilydetermined by those of ordinary skill in the art without undueexperimentation.

The hydride layer 16 provides the surface 12 of bond pad 10 withoxidation resistance. Yet, because the hydride layer is frangible,conventional ball or wedge wire bonding can be performed to obtainmetal-to-metal contact between surface 12 and the wire bonded thereto(not shown), which also provides a surface prepared for solderingoperations.

The hydride compound rapidly disintegrates during wire bonding orsoldering by two mechanisms. One mechanism is mechanical, and derivesfrom the frangibility of the hydride layer. The hydride will alsothermally de-hydride during bonding, forming a hydrogen cover over thebond pad itself, which also prevents oxidation.

It is not necessary for the hydride process to be performed at the timeof wafer fabrication. The hydride process can take place at any stageprior to wire bonding or soldering, so long as the hydrogen-containingatmosphere is sufficiently aggressive enough to reduce any contaminantsfrom the surface layer, and then subsequently hydride the surface layer.

The present invention also includes a single-step process in which thefrangible ceramic coating is not a metal hydride. Instead, a cleancopper bond pad is coated with a layer of a ceramic material having athickness that is suitable for soldering without fluxing and thatprovides the layer with a Rockwell Hardness (N-45) greater than about38, so that the layer is sufficiently frangible during ball or wedgewire bonding to obtain metal-to-metal contact between each bondingsurface and the wire bonded thereto.

Examples of suitable ceramic materials include nitrides and carbides ofsilicon, titanium and tantalium; oxides of aluminum, magnesium andzirconium; silicon and titanium dioxide; tungsten and boron carbide; andcubic boron nitride and diamond.

These coating layers are also formed by conventional vapor deposition oranalogous techniques.

The present invention can also be employed to coat the uninsulatedsurfaces of copper circuits other than the bond pads of semiconductors,using the same materials and method steps. Thus, the same ceramiccoatings can be used to protect prior to bonding the uninsulated coppercircuit surfaces of organic substrate packages such as Polymer Ball GridArrays (PBGA), Enhanced Polymer Ball Grid Arrays (EPBGA), Tape Ball GridArrays (TBGA), and the like; metal substrate packages such as Metal QuadFlat Packs (MQFP), Metal Leaded Chip Carriers (MLCC), Thin Small OutlinePackages (TSOP), and the like; and ceramic substrate packages such asCeramic Quad Flat Packs (CQFP), Ceramic Dual In-line Packages (CDIP),Leaderless Ceramic Chip Carriers (LCCC), and the like.

The present invention provides the uninsulated copper circuit portionsof electronic packages with oxidation-resistant surfaces that can beball or wedge wire bonded using conventional techniques without changesor additions to current ball and wedge wire bonding or flip chip bondingprocesses and equipment.

The following non-limiting example set forth hereinbelow illustratescertain aspects of the invention, but is not meant in any way torestrict the effective scope of the invention. All parts and percentagesare by weight unless otherwise noted, and all temperatures are in degreeCelsius.

EXAMPLE

Copper wafers having a copper thickness of at least 2,000 angstroms weremade via vapor deposition. Frangible ceramic coatings of silicon nitridewith thicknesses between 10 and 1,000 angstroms were formed viasputtering techniques.

Wire ball bonding was performed using various gold wires and a K&S Model8020 wire bonder. The following wire bond process conditions wereemployed:

-   -   Constant velocity=0.25-1.0 mil/msec.    -   Ultra sonic level=35-250 mAmp or equivalent power or voltage        setting    -   Bond time=5-50 msec.    -   Bond force=10-40 g    -   Free air ball diameter=1.4-3.0 mil

A variety of gold wire types were attempted and all were found to bereadily bondable: AFW-8, AFW-14, AFW-88, AFW-FP and AFW-FP2. The harderwires, AFW-FP and AFW-FP2, performed the best.

A variety of bonding tools (capillaries) were used and all were found toyield bondability in the bonded ball regions for which the capillarieswere designed. The best performing capillaries were part numbers414FA-2146-335 and 484FD-2053-335.

Copper wire was also bonded to the ceramic-coated bond pads. An inertcover gas was employed for ball formation. The bond parameters were notidentical for those of gold for the same bonded ball size, but the bondparameter range was not widely different for the range for gold ballbonding onto copper substrates.

The foregoing description of the preferred embodiments should be takenas illustrating, rather than as limiting, the present invention asdefined by the claims. Numerous variations and combinations of thefeatures set forth above can be utilized without departing from thepresently-claimed invention. Such variations should not be regarded as adeparture from the spirit and scope of the invention, and are intendedto be included within the scope of the following claims.

1. An electronic package comprising an uninsulated portion of a coppercircuit coated with a surface layer of a material selected from thegroup consisting of copper-rare earth metal complexes andcopper-immiscible metals that form metal hydride compounds, said surfacelayer having a thickness that, upon exposure to a reducing environmentcontaining hydrogen, forms a hydride layer having a thickness that issuitable for soldering without fluxing and that provides the layer witha hardness that is sufficiently frangible when being joined to anothermetal surface to obtain metal-to-metal contact between the surfaces. 2.The package of claim 1, wherein said surface layer comprises acopper-immiscible metal.
 3. The package of claim 2, wherein said surfacelayer is formed by co-deposition of said copper-immiscible metal andcopper to form said bond pad during wafer fabrication, followed byheating of said wafer so that said copper-immiscible metal migrates tosaid bond pad surface, thereby forming said surface layer.
 4. Thepackage of claim 3, wherein said surface layer is formed by vapor,electro-chemical deposition of said copper-immiscible metal onto saidbond surface.
 5. The package of claim 1, wherein said surface layerconsists essentially of a copper-rare earth metal complex.
 6. Thepackage of claim 5, wherein said copper complex is formed by vapordeposition, electrodeposition or chemical deposition of said rare earthmetal in a layer onto said bond pad surface.
 7. The package of claim 6,wherein said copper complex forms by heating said deposited rare earthmetal layer.
 8. The package of claim 5, wherein said rare earth metal isselected from the group consisting of La, Y and Ce.
 9. The package ofclaim 1, wherein said copper-immiscible metal is selected from the groupconsisting of Ta, V and Nb.
 10. The package of claim 1, wherein saidpackage comprises aan organic substrate package, a metal substratepackage or a ceramic substrate package
 11. The package of claim 1,comprising a semiconductor wafer including at least one device having anuninsulated copper bond pad.
 12. An electronic package comprising anuninsulated portion of a copper circuit coated with a surface layer of arare earth metal that forms a copper complex, said surface layer havinga thickness that, upon formation of said copper complex and exposure toa reducing environment comprising hydrogen, forms a hydride layer havinga thickness that is suitable for soldering without fluxing and that issufficiently frangible when being joined to another metal surface toobtain metal-to-metal contact between the surfaces.
 13. The package ofclaim 12, wherein said surface layer is formed by vapor deposition,electrodeposition or chemical deposition of said rare earth metal in alayer on said bond pad surface.
 14. The package of claim 12, whereinsaid rare earth metal is selected from the group consisting of La, Y andCe.
 15. The package of claim 12, wherein said package comprises anorganic substrate package, a metal substrate package or a ceramicsubstrate package.
 16. The package of claim 12, comprising asemiconductor wafer including at least one device having an uninsulatedcopper bond pad.